1. Field of the Invention
The present invention relates to an optical record and reproduction apparatus for detecting a tracking error signal, and more particularly to an optical record and reproduction apparatus having excellent economical efficiency and reliability.
2. Description of the Background Art
In a conventional optical record and reproduction apparatus, signals are recorded or reproduced in a spiral or concentric circular form to or from an information recording medium in a non-contact manner, and thus a tracking servo sensor system is required. Concerning the tracking servo sensor system, various systems have been proposed, for instance, a push-pull method utilizing a diffraction light beam reflected by a signal pit or guide groove formed on the information recording medium, or a three beam method using two supplementary beams.
In the former push-pull method, a diffraction light distribution formed by a guide groove of the information recording medium is projected to a dividing light detector to obtain a tracking error signal from its differential output. In the latter three beam method, the light beam is divided into a plurality of light beams by using a beam divider means such as a diffraction grating or the like, and a tracking error signal is obtained from a differential output of two supplementary beams positioned on both sides of a main beam for use in recording and reproducing.
These two methods are well-known, but there exist the following problems.
In the push-pull method, since a guide groove is used, influence of the guide groove depth is given. FIG. 1 shows the relationship between a push-pull signal and a guide groove depth in the push-pull method. As is apparent from FIG. 1, the push-pull signal value is the maximum or the minimum when the groove depth is .lambda./8 or .lambda./4, respectively. Accordingly, it is a disadvantageous system for a CD (compact disk) having a pit depth of approximately .lambda./4.
Meanwhile, in the three beam method, since the supplementary beams are used for recording signals onto a W/O (write once) disk having a groove depth of approximately .lambda./8, the recording signal is disrupted by the supplementary beams or an offset is caused.
FIGS. 2A and 2B show a conventional information recording medium having a recording track or a guide groove 17 and land portions 18, on which a main light beam 19 and first and second supplementary light beams 20a and 20b are focused, and the guide groove 17 includes record portions 21 recorded by the main beam 19. As shown in FIGS. 2A and 2B, the main beam 19 is tracked on the guide groove 17. In the case of FIG. 2B, the first and second supplementary beams 20a and 20b are both focused on the recording portions 21, and the same signal is picked up from the first and second supplementary beams 20a and 20b. Hence, a differential output of the supplementary beams 20a and 20b is zero, and no offset is caused. On the other hand, in the case of FIG. 2A, the first supplementary beam 20a is not focused on the recording portion 21 but the second supplementary beam 20b is focused on the recording portions. Hence, the outputs of the first and second supplementary beams 20 a and 20b are different from each other, and an offset is caused.
As described above, both the conventional methods involve problems. In a conventional optical head device for recording and reproducing the signals onto and from the information recording medium such as the CD and the W/O disk, both tracking servo systems are switched depending on the kinds of the disks, and it is necessary to mechanically insert or remove the beam divider means such as the diffraction grating into or out of the light beam path.
In FIG. 3, there is shown an optical system of a conventional optical head device. A light beam generated by a semiconductor laser 1 is focused onto an information recording medium 6 through a diffraction grating 2, a collimator lens 3, a beam splitter 4 and an objective lens 5. The light beam reflected by the information recording medium 6 is then concentrated into a light detector 8 through the objective lens 5, the beam splitter 4 and a condenser lens 7.
The operation of this optical system will now be described. The diffraction grating 2 is arranged to be movable into or out of the path of the light beam generated by the semiconductor laser 1. Hence, when the signal is recorded onto the W/O disk, the diffraction grating 2 is retracted from the light path, and the tracking error signal is obtained by the push-pull method. On the other hand, when the signal stored in the W/O disk or CD is read out and reproduced, the diffraction grating 2 is moved into the light path, and the tracking error signal is obtained by the three beam method. That is, by moving the diffraction grating 2 into or out of the path of the light beam generated by the semiconductor laser 1, an optimum tracking error signal is obtained in a tracking servo sensor system. In this case, the tracking error signal is obtained by the push-pull method using one beam for the W/O disk or by the three beam method for the CD.
In FIG. 4, there is shown another conventional optical record and reproduction apparatus for obtaining a tracking error signal by a push-pull method using three beams and by using the three beam method in both the recording and reproducing operations. A semiconductor laser 1 projects a laser light beam B to an information recording medium 6 such as an optical disk via a collimator lens 3, a diffraction grating 2 for dividing the parallel light beam passed through the collimator lens 3 into a zero-order light (first beam B1) and .+-. primary lights (second and third beams B2 and B3), a polarization beam splitter 4 for passing the first to third light beams, a reflector mirror 19, a 1/4 wavelength plate 22 and an objective lens 5. A driver circuit 21 drives the semiconductor laser 1 as hereinafter described in detail. The reflector mirror 19, the 1/4 wavelength plate 22 and the objective lens 5 are arranged between the polarization beam splitter 4 and the information recording medium 6. The diffraction grating 2 may be positioned between the semiconductor laser 1 and the collimator lens 3.
The information recording medium 6 is provided with an information track or tracks T extending in the turning direction of the information recording medium 6, as indicated by an arrow in FIG. 5 or FIG. 6. On the information track T, the first to third beams B1 to B3 are irradiated, and, in particular, a plurality of pits P are recorded or reproduced by a first light spot S1 formed by the first beam B1 positioned between the second and third beams B2 and B3. Hence, the first spot S1 of the first beam B1 is always positioned in the central position of the information track T. On the other hand, on reproducing, as shown in FIG. 5, since the second and third beams B2 and B3 respectively precede and follow the first beam B1, they contribute as the supplementary beams to the production of the tracking error signal, and thus second and third light spots S2 and S3 formed by the respective second and third beams B2 and B3 are placed in positions shifted from the central position of the information track T. On recording, as shown in FIG. 6, since the second and third beams monitor the situation of the information track T, the second and third light spots S2 and S3 are positioned in the central position of the information track T. In the recording, the intensity of the first beam B1 is strong, and, since the second and third beams B2 and B3 only contribute to the monitoring of the state of the information track T, the intensity of the second and third beams B2 and B3 is controlled so as to be sufficiently weak.
A slant mechanism 20 is provided for the diffraction grating 2 for slanting the diffraction grating 2 a certain minute angle .theta. through a drive mechanism (not shown) in response to a switching signal C fed to the slant mechanism 20 from a signal switch circuit 13 so as to switch the directions of the first to third beams B1, B2 and B3. That is, the slant mechanism 20 constitutes a light spot switch means for switching positions of second and third spots S2 and S3 on an information track T of the information recording medium 6 depending on whether a recording or a reproducing operation is being carried out.
In this instance, the semiconductor laser 1, the diffraction grating 2, the collimator lens 3, the polarization beam splitter 4, the objective lens 5, the reflector mirror 19 and the slant mechanism 20 constitute a beam irradiation device for irradiating the first to third beams on the information track T of the information recording medium 6.
A group of sensor lenses 7 and 10 such as convex and cylindrical lenses, respectively, concentrate the first to third beams B1, B2 and B3 reflected by the polarization beam splitter 4 to a light detector 8 having six divided sensing surfaces 8a to 8f, as shown in FIG. 7, (four central surfaces 8a to 8d for receiving the first light spot S1' formed by the reflected first beam B1 and two side surfaces 8e and 8f for receiving the second and third spots S2' and S3' formed by the reflected second and third beams B2 and B3) so as to detect the first to third beams independently.
In FIG. 4, a signal Dad has a luminous energy corresponding to the sum of signals Da and Dd having respective luminous energy, output from the sensing surfaces 8a and 8d of the light detector 8, and a signal Dbc has a luminous energy corresponding to the sum of signals Db and Dc having respective luminous energy, output from the sensing surfaces 8b and 8c of the light detector 8. A signal De has a luminous energy corresponding to the intensity of the reflected second light beam B2, output from the sensing surface 8e of the light detector 8, and a signal Df has a luminous energy corresponding to the intensity of the reflected third light beam B3, output from the sensing surface 8f of the light detector 8.
The signal switch circuit 13 controls a link switch 12 and outputs a control signal thereto in order to switch the detecting systems for obtaining a tracking error signal E. The signal switch circuit 13 also outputs the switching signal C to the slant mechanism 20, as described above. In response to the control signal sent from the signal switch circuit 13, the link switch 12 selects one of pairs of signals Dad and De or signals Dbc and Df at the same time to output the selected signals to input terminals of a differential amplifier 14, and the differential amplifier 14 outputs the tracking error signal E to a tracking error signal detector 15 depending on the difference between the two signals selected by the link switch 12. A prerecord monitor 16 receives the signal De and detects a prerecord state of the information track T according to the signal De, and a postrecord monitor receives the signal Df and detects a postrecord state of the information track T according to the signal Df. A record and reproduction control unit 18 receives output signals fed from the prerecord and postrecord monitors 16 and 17 and detects information from the signals such as recording states, sector numbers and so forth.
The operation of the conventional record and reproduction apparatus will now be described in detail with reference to FIGS. 4 to 7.
First, when the information stored in the information recording medium 6 is reproduced, the light beam B emitted by the semiconductor laser 1 is formed into the parallel light beam by the collimator lens 3, and then the parallel light beam is passed through the diffraction grating 2 where it is divided into the zero-order first beam B1 projecting in the same direction as the incident parallel light beam and the .+-. primary second and third light beams B2 and B3 projecting in a somewhat deflected manner with respect to the incident parallel light beam. Then, the first to third light beams B1 to B3 are irradiated onto the information recording medium 6 via the polarization beam splitter 4, the reflector mirror 19, the 1/4 wavelength plate 22 and the objective lens 5 to form the first to third light spots S1 to S3 on the information track T of the information recording medium 6, as shown in FIG. 5.
In this instance, the light intensities of the beams B1 to B3 are not modulated and thus are fixed output values. The light intensity ratio of the first beam B1 to the second or third beam B2 or B3 is predetermined by the designing of the diffraction grating 2. Also, the first light spot S1 of the first light beam B1 is irradiated in the central position of the information track T while the second and third light spots S2 and S3 of the second and third light beams B2 and B3 functioning as the supplementary beams for detecting the tracking error signal E are irradiated out of the central position of the information track T.
Then, the light beams B1 to B3 reflected by the information recording medium 6 are passed again through the objective lens 5 and the 1/4 wavelength plate 22 to reach the polarization beam splitter 4. Since the polarization direction of the light beams B1 to B3 is rotated 90 degree when the light beams B1 to B3 pass twice through the 1/4 wavelength plate 22, the returned light beams B1 to B3 are reflected by the polarization beam splitter 4 in a perpendicular direction to the original light beam. The light beams B1 to B3 reflected by the polarization beam splitter 4 are concentrated onto the light detector 8 via the sensor lenses 7 and 10 to form the first to third light spots S1' to S3' on the six divided sensor surfaces 8a to 8f of the light detector 8, as shown in FIG. 7.
In this case, since the first light spot S1' of the first beam B1 includes the recording information of the pits formed on the information track T of the information recording medium 6, the reproduction signals can be obtained by calculating the sum of the signals Da to Dd output from the sensor surfaces 8a to 8d on which the first light spot S1' is projected, in a conventional manner.
The signals Da to Df having the luminous energy detected on the sensor surfaces 8a to 8f of the light detector 8 are used for the calculation on the basis of a conventional signal detecting method, and then a focusing error signal and a tracking error signal are obtained by the astigmatic method and the three beam method, respectively.
In the reproducing operation, the link switch 12 is turned from the position shown in FIG. 4 to the other position by the signal switch circuit 13, and the signals De and Df output from the sensor surfaces 8e and 8f of the light detector 8 are selected. Hence, the tracking error signal output by the differential amplifier 14 is represented by the difference between the signals De and Df as follows. EQU E=De-Df
The obtained tracking error signal E is input to the tracking error signal detector 15 which discriminates whether or not the tracking error signal is correct. The signals De and Df are fed to the prerecord and postrecord monitors, respectively, but the monitorings are not carried out in the reproducing operation described above.
On the other hand, when the information is recorded on the information recording medium 6, the driver circuit 21 performs a pulse drive of the semiconductor laser 1 in accordance with the recording information to emit the light beam B including the recording information, for example, the information corresponding to the pulse width. The light beam B is then divided into three light beams B1, B2 and B3 and the three beams B1 to B3 are focused in the form of the light spots S1 to S3 onto the information track T of the information recording medium 6 in the same manner as the reproducing operation. In this case, the signal switch circuit 13 sends the switching signal C to the slant mechanism 20 in order to control it and rotates the diffraction grating 2 to a certain angle .theta. so that the second and third light spots S2 and S3 may be placed in the central position of the information track T of the information recording medium 6, as shown in Fig. 6.
As this time, the intensity of the first light beams B1 for recording is determined as strong, and the intensity of the second and third light beams B2 and B3 is determined as being sufficiently weak as compared with that of the first light beam B1 so as not to affect the recording.
The first light spot S1 successively forms the pits having shapes corresponding to the recording information on the information track T of the information recording medium 6 and is reflected thereby at the same time. The second light spot S2 preceding the first light spot S1 is reflected by the information track T on which no pit is recorded, and the third light spot S3 following the first light spot S1 is reflected by the information track T on which the pits are recorded.
Then, the first to third light beams B1 to B3 reflected by the information recording medium 6 are projected onto the light detector 8 in the form of the first to third light spots S1' to S3' in the same manner as the reproducing operation described above. In this instance, the tracking error signal is obtained by using the push-pull method. At this time, the link switch 12 is selected to the position shown in FIG. 4 by the signal switch circuit 13 to select the signals Dad and Dbc. Hence, the tracking error signal E is obtained as follows. EQU E=(Da+Dd)-(Dd+Dc)=Dad-Dbc
In this case, the second and third beams B2 and B3 are not used for the detection of the tracking error signal E, and the signals De and Df are input to the prerecord and postrecord monitors 16 and 17, respectively, for use in detecting the state of the information track T before and after the recording of the information thereon. That is, by using the reflected second light beam B2, it is checked whether or not there is a defect on the information track T before the recording, and by using the reflected third light beam B3, a check is carried out whether or not the pits have been correctly recorded on the information track T after the recording.
Thus the obtained monitoring signals output from the prerecord and postrecord monitors 16 and 17 are fed to the record and reproduction control unit 18 where, when it is discriminated that there is a defect, processing such as another recording or the like is conducted to improve the recording error rate on the information recording medium 6.
As described above, in the conventional optical record and reproduction apparatuses, by moving the beam divider means such as the diffraction grating into or out of the light beam path or rotating the diffraction grating at a certain angle within the light beam path to change the direction of the light spots, the tracking error signal is obtained by using the push-pull method or the three beam method. Hence, in the conventional optical record and reproduction apparatuses, it is required to mechanically move or rotate the diffraction grating, and, since the position of the light beam with reference to the central position of the information track is required with an accuracy of a submicron order, problems arise. For example, the driving mechanism for the diffraction grating is complicated, and the responsive ability is bad. Further, the cost of the driving mechanism increases. Also, when the determined angle of the diffraction grating is shifted from the designed value, the amplitude of the monitor signal or the tracking error signal is reduced.